commit.h: drop redundant comment

We mention twice that the from_ident field of struct
pretty_print_context is internal.

The first comment was added by 10f2fbf, which prepares the
struct for internal fields, and then the second by a908047,
which actually adds such a field. This was a mistake made
when re-rolling the series on the list; the comment should
have been removed from the latter commit.

Signed-off-by: Jeff King <peff@peff.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>